GCOS stands for "GameCube Operating System" and was conceived in 2004 by emu_kidid and ShadowSwan.
It started off by taking the publicly released IPL Replacement code that tmbinc had made and merged together the (at the time) recently reversed ViperGC drive patching methods found in the Cobra BIOS called 'Anaconda04'. Initial versions of GCOS started out with just a text UI, but eventually the project gained approximately 15 contributors across its life, making it a well known piece of software for the GameCube. The successor to GCOS is Swiss, which was re-written from scratch, but borrows some small bits of code from GCOS.
After the Wii was released, the GCOS source base was used as a way of creating a menu for multi-game compilation discs for Wii. A GUI program for Windows was also created for this purpose which was known as 'MultiGame ISO Creator', ending at v4.0.4F+.
Binary and Source downloads available at GC-Forever: http://www.gc-forever.com/forums/viewtopic.php?f=4&t=4